A surveying apparatus is equipped with a rotary encoder for measuring a horizontal angle and a vertical angle. The rotary encoder is a device that measures a horizontal angle and a vertical angle by a rotary disk that rotates in conjunction with a telescope and a detector that reads a scale provided along the circumferential direction of this rotary disk.
Rotary encoders include incremental encoders and absolute encoders. An incremental encoder is provided with a rotary disk with an equal-interval and equal-width scale (slits). This incremental encoder irradiates light emitted from a light source onto the rotary disk and detects brightness/darkness according to a number of the scale by a photodetector so as to measure a rotation angle of the rotary disk.
An absolute encoder is, for example, as shown in FIG. 1, provided with a rotary disk 1 applied with slits composed of narrow slits that mean 0 and wide slits that mean 1 and forming, for example, angle codes 11 that indicate angles along a periphery thereof. This absolute encoder irradiates light emitted from a light source (LED) 2 emission-controlled by a CPU 6 onto the rotary disk 1 and detects the angle code 11 with a linear sensor 3. An output of the CCD linear sensor 3 is inputted to the CPU 6 via an A/D converter 5. The CPU 6 decodes the angle code 11 and applies an appropriate interpolation process to thereby calculate an absolute angle of the rotary disk 1 and display the absolute angle on the display unit 7 (see the following Patent Document 1.)
However, in the conventional rotary encoder used for a surveying apparatus, since it is difficult to attach the rotary disk 1 to a rotating shaft without eccentricity and to provide the scale and angle codes completely at equal intervals, errors have occurred in measured values. In order to eliminate such errors, self-calibration by a pair of measurements has been carried out in accordance with the ISO and JSIM standards. For this self-calibration, collimators are disposed around the surveying apparatus, the collimators are collimated while rotating the surveying apparatus by an appropriate angle, and a scale or angle code error is measured.
Patent Document 1: Japanese Published Unexamined Patent Application No. 2002-13949